Motion in a joint moves perpendicular to the axis. For example reference the following image.
The subtalar and sacroiliac (SI) joint share a very interesting property. The axis appears to be similar with the subtalar joint highly utilized in the frontal and transverse planes.
The sacroiliac joint also is highly utilized in the frontal and transverse planes as evidenced by an oblique axis anterior to posterior like the subtalar joint.
Therefore given a “normal” subtalar joint, without ‘energy leaks up the kinetic chain’, a 1:1 ratio of frontal to transverse plane motion would reach the SI joint asking for a similar response. Everything in this scenario would be happier than the following.
The axis of the subtalar joint in the above picture has changed from “normal”. The axis now has allowed for greater frontal plane motion than transverse. Let’s say this ratio is now 2:1 frontal plane to transverse plane. A subtalar joint with a 2:1 ratio now communicates to the sacroiliac joint (SI) joint asking for a 2:1 ratio of frontal to transverse plane. The sacroiliac joint does not have much motion to give ~4 degrees. Therefore it will begin to steal motion from above or below itself while being abused by moving contrary to it’s 1:1 axis. Therefore we now have SI dysfunction from the subtalar joint.